Polymeric film having a coating layer of a phosphonic acid group containing polymer

ABSTRACT

A polymeric film which is substantially gelatin free has a polymeric film substrate and a coating layer containing a polymer having at least one or more repeating units having at least one or more pendant (—POXY) groups, wherein X and Y, which may be the same or different, are OH or OM wherein M is a cation. The polymeric film is suitable for use as a component of a printing plate.

[0001] This invention relates to a polymeric film, and in particular toa polymeric film which is substantially gelatin free and has a coatinglayer which is suitable for use as a component of a printing plate.

[0002] Printing plates, particularly lithographic printing plates,generally comprise a substrate, a hydrophilic coating layer, and aphotopolymerizable light-sensitive layer. Imagewise exposure, by asuitable light source, results in hardening of the photopolymerizablelayer, allowing the unhardened portions of the layer to be removed bywashing with a solvent. The result is a hydrophobic polymer image on ahydrophilic substrate, which can be used as a lithographic printingplate. In an alternative process, a hydrophobic toner is applied to thehydrophilic coating layer by means of a laser imaging process.

[0003] The surface properties of the hydrophilic coating layer can becrucial in determining the quality of the final printed image. Forexample, some prior art hydrophilic coating layers exhibit poor coatquality and insufficient adhesion to the underlying substrate and/oroverlying photopolymerized layer or toner. In addition, the coatinglayer may possess insufficient hydrophilicity and/or a surfacetopography which can lead to inadequate removal of the unhardenedphotopolymerized portions, resuming in the formation of a relativelypoor quality printed image. In laser toner based processes thehydrophilic coating layer may require antistatic properties to controlor avoid toner scatter, which reduces the quality of the final image.

[0004] Relatively high temperatures are employed in some processes usedfor producing printing plates, which can affect the curl and flatness ofany polymeric film present in a printing plate.

[0005] Hydrophilic coating layers such as gelatin are traditionallyapplied to a polymeric film after the production of the film has beencompleted, i.e., “off-line”, which results in an increase in the numberof process steps required to produce the coated film. There is a need tobe able to apply the coating layer during the film making process, i.e.,“in-line”, without the use of gelatin in order to simplify and improvethe efficiency of the production process.

[0006] We have now devised a polymeric film which reduces orsubstantially overcomes at least one of the aforementioned problems.

[0007] Accordingly, the present invention provides a polymeric filmwhich is substantially gelatin free and comprises a polymeric filmsubstrate having on at least one surface thereof, a coating layercomprising a polymer comprising at least one or more repeating unitscomprising at least one or more pendant (—POXY) groups, wherein X and Y,which may be the same or different, are OH or OM wherein M is a cation.

[0008] The invention further provides a method of producing a polymericfilm which comprises forming a polymeric film substrate, applying acoating composition to at least one surface of the substrate, thecoating composition comprising a polymer comprising at least one or morerepeating units comprising at least one or more pendant (—POXY) groups,wherein X and Y, which may be the same or different, are OH or OMwherein M is a cation.

[0009] The invention also provides a printing plate comprising apolymeric film substrate having on at least one surface thereof, acoating layer comprising a polymer comprising at least one or morerepeating units comprising at least one or more pendant (—POXY) groups,wherein X and Y, which may be the same or different, are OH or OMwherein M is a cation.

[0010] The polymeric film according to the invention can also be fusedas coating for mirrors, in particular car mirrors, and building surfacecladding. The polymeric film substrate is a film capable of independentexistence in the absence of a supporting base.

[0011] The substrate, to which a coating layer composition is applied toyield a polymeric film according to the invention, may be formed fromany suitable film-forming, polymeric material. Thermoplastics materialsare preferred, and include a homopolymer or copolymer of a 1-olefin,such as ethylene, propylene and but-1-ene, a polyamide, a polycarbonate,more preferably a polyester, and particularly a synthetic linearpolyester which may be obtained by condensing one or more dicarboxylicacids or their lower alkyl (up to 6 carbon atoms) diesters, e.g.,terephthalic acid, isophthalic acid, phthalic acid, 2,5-, 2,6- or2,7-naphthalenedicarboxylic acid, succinic acid, sebacic acid, adipicacid, azelaic acid, 4,4′-diphenyldicarboxylic acid,hexahydro-terephthalic acid or 1,2-bis-p-carboxyphenoxyethane(optionally with a monocarboxylic acid, such as pivalic acid) with oneor more glycols, particularly aliphatic glycols, e.g., ethylene glycol,1,3-propanediol, 1,4-butanediol, neopentyl glycol and1,4-cyclohexane-dimethanol. A polyethylene terephthalate and/orpolyethylene naphthalate film is preferred. A polyethylene terephthalatefilm is particularly preferred, especially such a film which has beenbiaxially oriented by sequential stretching in two mutuallyperpendicular directions, typically at a temperature in the range from70 to 125° C., and preferably heat set, typically at a temperature inthe range from 150 to 250° C., for example as described in GB-A-838708.Another preferred film comprises a copolymer of terephthalic andisophthalic acids with ethylene glycol. The substrate may also comprisea polyarylether or thio analogue thereof, particularly apolyaryletherketone, polyarylethersulphone, polyaryletherether-ketone,polyaryletherethersulphone, or a copolymer or thioanalogue thereof.Examples of these polymers are disclosed in EP-A-1879, EP-A-184458 andU.S. Pat. No. 4,008,203. Blends of these polymers may also be employed.A poly p-phenylene sulphide film is also suitable.

[0012] Suitable thermoset resin substrate materials includeaddition-polymerization resins, such as acrylics, vinyls, bis-maleimidesand unsaturated polyesters, formaldehyde condensate resins such ascondensates with urea, melamine or phenols, cyanate resins, isocyanateresins, epoxy resins, functionalized polyesters, polyamides orpolyimides.

[0013] A film substrate for a polymeric film according to the inventionmay be unoriented or preferably oriented, for example uniaxiallyoriented, or more preferably biaxially oriented by drawing in twomutually perpendicular directions in the plane of the film to achieve asatisfactory combination of mechanical and physical properties.Formation of the film may be effected by any process known in the artfor producing a polymeric film, for example a tubular or a flat filmprocess.

[0014] In a tubular process simultaneous biaxial orientation may beeffected by extruding a thermoplastics polymeric tube which issubsequently quenched, reheated and then expanded by internal gaspressure to induce transverse orientation, and withdrawn at a rate whichwill induce longitudinal orientation.

[0015] In the preferred flat film process a film-forming polymer isextruded through a slot die and rapidly quenched upon a chilled castingsurface (drum) to ensure that the polymer is quenched to the amorphousstate. Orientation is then effected by stretching the quenched extrudatein at least one direction at a temperature above the glass transitiontemperature of the polymer. Sequential orientation may be effected bystretching a flat, quenched extrudate firstly in one direction, usuallythe longitudinal direction, i.e. the forward direction through the filmstretching machine, and then in the transverse direction. Forwardstretching of the extrudate is conventionally effected over a set ofrotating rolls or between two pairs of nip rolls, transverse stretchingthen being effected in a stenter apparatus. Stretching is effected to anextent determined by the nature of the film-forming polymer, for examplea polyester is usually stretched so that the dimension of the orientedpolyester film is from 2.5 to 4.5 its original dimension in the, oreach, direction of stretching.

[0016] A stretched film may be, and preferably is, dimensionallystabilized by heat-setting under dimensional restraint at a temperatureabove the glass transition temperature of the film-forming polymer butbelow the melting temperature thereof, to induce crystallization of thepolymer.

[0017] In a preferred embodiment, and in order that the polymeric filmof the present invention has a low distortion, reduced curl and improvedflatness (or cockle), the polymeric film has a percentage thermalexpansion in the film widthwise direction (TD) at 150° C. of 0.01 to1.0%, and a percentage thermal shrinkage in the film lengthwisedirection (MD) at 150° C. of 0.4 to 2.0%. Preferably the film exhibits aTD expansion at 150° C. of 0.2 to 0.8%, and a MD shrinkage at 150° C. of0.5 to 1.5%, and particularly a TD expansion at 150° C. of 0.3 to 0.5%,and a MD shrinkage at 150° C. of 0.7 to 1.0%.

[0018] The substrate of a polymeric film of the present invention can beprepared, for example as described above, during the production of abiaxially drawn film. In a typical process for the production of abiaxially drawn film, the film is preferably firstly stretched in thelongitudinal direction over a series of rotating rollers, and thenstretched transversely in a stenter oven, preferably followed by heatsetting under tension in the stenter apparatus. The tension in thewidthwise direction can be provided by clips which hold the film, theclips being attached to parallel rails on opposite sides of the stenterapparatus. The tension in the widthwise direction can be reduced orremoved, for example by moving the rails inwards towards the exit end ofthe stenter - this is known as “toe-in”. By employing toe-in it ispossible to allow the film to shrink to a certain degree, and by thismeans obtain film with the required TD expansion and MD shrinkagecharacteristics. The amount of toe-in employed, for example in theproduction of a polyethylene terephthalate film should be 0.1 to 10%,preferably 3 to 7%, and particularly 3.5 to 6%. The exact amount oftoe-in required will depend upon the particular film being produced, andupon the other process conditions being used. It is preferred that thestenter is operated at relatively high temperatures, for example forpolyethylene terephthalate film the stenter temperature is suitably 230to 245° C., particularly 235 to 240° C.

[0019] In one embodiment of the invention the polymeric film istransparent, exhibiting high optical clarity and low haze, preferablyhaving a wide angle haze, being measured according to the standard ASTMD 1003-61, of <8%, more preferably <6%, particularly <5%, and especially<3%, preferably for a 175 μm thick film. The aforementioned opticalcharacteristics can be suitably achieved by having little or noparticulate additive present in the substrate. The substrate may containrelatively small quantities of filler material, for example in the rangefrom 5 to 3000 ppm, preferably 50 to 2000 ppm, and more preferably 100to 1000 ppm. Suitable fillers include inorganic materials such assilica, china clay, calcium carbonate, and organic materials such assilicone resin particles. Spherical monodisperse fillers may beemployed. The substrate may contain filler due to the normal practice ofusing reclaimed film in the film manufacturing process.

[0020] However, in a further embodiment of the invention the polymericfilm is opaque, which is defined as a film exhibiting a TransmissionOptical Density (Sakura Densitometer; type PDA 65; transmission mode) offrom 0.75 to 1.75, and particularly of from 1.2 to 1.5, preferably for a175 μm thick film. The polymeric film is conveniently rendered opaque byincorporating into the synthetic polymer of the substrate layer, aneffective amount of an opacifying agent. Suitable opacifying agentsinclude a particulate inorganic filler, an incompatible resin filler, ora mixture of two or more such fillers.

[0021] The polymeric film may also be translucent, i.e. having atransmission optical density of up to 0.75.

[0022] Particulate inorganic fillers suitable for generating an opaquefilm substrate include conventional inorganic pigments and fillers, andparticularly metal or metalloid oxides, such as alumina, silica andtitania, and alkaline metal salts, such as the carbonates and sulphatesof calcium and barium. Suitable inorganic fillers may be homogeneous andconsist essentially of a single filler material or compound, such astitanium dioxide or barium sulphate alone. Alternatively, at least aproportion of the filler may be heterogeneous, the primary fillermaterial being associated with an additional modifying component. Forexample, the primary filler particle may be treated with a surfacemodifier, such as a pigment, soap, surfactant coupling agent or othermodifier to promote or after the degree to which the filler iscompatible with the outer layer polymer.

[0023] Suitable particulate inorganic fillers may be of the non-voidingor voiding type, i.e. by voiding is meant comprises a cellular structurecontaining at least a proportion of discrete, closed cells. Bariumsulphate is an example of a filler which results in the formation ofvoids. Titanium dioxide may be of the voiding or non-voiding type,dependent upon the particular type of titanium dioxide employed. In apreferred embodiment of the invention, the film substrate comprisestitanium dioxide particles, more preferably of the non-voiding type.

[0024] The amount of inorganic filler incorporated into the filmsubstrate desirably should be not less than 2% nor exceed 40% by weight,based on the weight of the substrate polymer. Particularly satisfactorylevels of opacity are achieved when the concentration of filler,suitably titanium dioxide, is preferably in the range from 5% to 25%,more preferably 8% to 18%, and particularly 11% to 14% by weight, basedon the weight of the substrate polymer.

[0025] The inorganic filler particles preferably have a volumedistributed median particle diameter (equivalent spherical diametercorresponding to 50% of the volume of all the particles, read on thecumulative distribution curve relating volume % to the diameter of theparticles—often referred to as the “D(v,0.5)” value) in the range from0.2 to 1.5 μm, more preferably 0.4 to 1.1 μm, particularly 0.6 to 0.8μm, and especially 0.65 to 0.75 μm.

[0026] The preferred titanium dioxide particles may be of anatase orrutile crystal form. The titanium dioxide particles preferably comprisea major portion of anatase, more preferably at least 60%, particularlyat least 80%, and especially approximately 100% by weight of anatase.The particles can be prepared by standard procedures, such as using thechloride process or preferably by the sulphate process.

[0027] In one embodiment of the invention the titanium dioxide particlesare coated preferably with inorganic oxides of elements such asaluminum, silicon, zinc, magnesium or mixtures thereof. Preferably thecoating additionally comprises an organic compound, such as fatty acidsand preferably alkanols, suitably having from 8 to 30, preferably from12 to 24 carbon atoms. Polydiorganosiloxanes orpolyorganohydrogensiloxanes, such as polydimethylsiloxane orpolymethylhydrogensiloxane are suitable organic compounds.

[0028] The coating is suitably applied to the titanium dioxide particlesin aqueous suspension. The inorganic oxides are precipitated in aqueoussuspension from water-soluble compounds such as sodium aluminate,aluminium sulphate, aluminium hydroxide, aluminium nitrate, silicic acidor sodium silicate.

[0029] The individual or primary titanium dioxide particles suitablyhave a mean crystal size, as determined by electron microscopy, in therange from 0.05 to 0.4 μm, preferably 0.1 to 0.3 μm, and more preferably0.2 to 0.25 μm. In a preferred embodiment of the invention, the primarytitanium dioxide particles aggregate to form clusters or agglomeratescomprising a plurality of titanium dioxide particles. The aggregationprocess of the primary titanium dioxide particles may take place duringthe actual synthesis of the titanium dioxide and/or during the polymerand/or polymer film making process.

[0030] The film substrate optionally comprises an “incompatible resin”by which is meant a resin which either does not melt, or which issubstantially immiscible with the substrate polymer, at the highesttemperature encountered during extrusion and fabrication of the layer.Such resins include polyamides and olefin polymers, particularly a home-or co-polymer of a mono-alpha-olefin containing up to 6 carbon atoms inits molecule, for incorporation into polyester films, or polyesters ofthe kind hereinbefore described for incorporation into polyolefin films.

[0031] The amount of incompatible resin, preferably polyolefin,incorporated into the film substrate is preferably in the range from 1%to 15%, more preferably 3% to 10%, and particularly 5% to 8% by weight,based on the weight of the substrate polymer.

[0032] Incorporation of the opacifying agent, preferably inorganicfiller, into the substrate layer polymer may be effected by conventionaltechniques, for example by mixing with the monomeric reactants fromwhich the polymer is derived, by dry blending with the polymer ingranular or chip form prior to formation of a film therefrom, or byusing masterbatching technology.

[0033] By a pendant (—POXY) group of a repeating unit(s) of the coatinglayer polymer is meant a group which is not part of the backbone chainof the polymer, i.e. the group is present in a side chain attached tothe backbone chain of the polymer. X and Y, which may be the same ordifferent, are OH or OM wherein M is a cation. M may be a metal ion,preferably an alkali metal ion, more preferably Li⁺, Na⁺ or K⁺, or aquaternary ammonium ion. Both X and Y are preferably OH groups, i.e. thepreferred pendant group is

[0034] Thus, the coating layer polymer preferably comprises repeat unitscontaining pendant phosphoric acid groups and/or salts or otherderivatives thereof.

[0035] Suitable repeating units are derived during the polymerization ofmonoethylenically unsaturated monomers containing phosphonic acidgroups, which may be aromatic, heterocyclic, aliphatic andcycloaliphatic. Preferred monomers include vinyl phosphonic acid,divinyl phosphonic acid, allyl phosphonic acid, methallyl phosphonicacid, vinyl phosphonic acid monomethyl ester, methacrylamidomethanephosphonic acid, 2-arylamido-2-methylpropane phosphonic acid,3-phosphonopropyl acrylate and 3-phosphonopropyl methacrylate. Vinylphosphoric acid is a particularly preferred monomer.

[0036] The coating layer polymer suitably comprises greater than 5 mole%, preferably in the range from 10 to 90 mole %, more preferably 30 to80 mole %, particularly 45 to 75 mole % and especially 50 to 70 mole %of repeating units comprising phosphoric acid containing monomer asherein described.

[0037] The coating layer polymer is preferably a copolymer, comprisingone or more, preferably acrylic, comonomers, in addition to therepeating units as herein described. Suitable additional comonomers maybe selected from acrylic acid, methacrylic acid or a derivative ofacrylic acid or methacrylic acid, preferably an ester of acrylic acid ormethacrylic acid, especially an alkyl ester where the alkyl groupcontains up to ten carbon atoms such as methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl, terbutyl, hexyl, 2-ethylhexyl, heptyl, andn-octyl. An alkyl acrylate, e.g., ethyl acrylate or butyl acrylate,and/or an alkyl methacrylate, e.g, methyl methacrylate, may be employed.

[0038] In a preferred embodiment of the present invention, the coatinglayer polymer additionally comprises repeating units containing pendantcarboxyl groups, or groups which will form carboxyl groups onhydrolysis. Suitable carboxyl group-containing repeating groups arederived during the polymerization of monomers such as acrylic acid,methacrylic acid, itaconic acid, maleic acid, maleic anhydride, orderivatives thereof. Acrylic acid is a particularly preferred carboxylgroup-containing comonomer.

[0039] The coating layer polymer preferably comprises in the range from10 to 70 mole %, more preferably 30 to 60 mole %, particularly 30 to 55mole % and especially 30 to 50 mole % of repeating units containingpendant carboxyl groups as herein described.

[0040] Other comonomers which are suitable for use in the preparation ofthe coating layer copolymer include acrylonitrile, methacrylonitrile,halo-substituted acrylonitrile, halo-substituted methacrylonitrile,hydroxyethyl methacrylate, glycidyl acrylate, glycidyl methacrylate,itaconic acid, itaconic anhydride and half esters of itaconic acid.Other optional comonomers include vinyl esters such as vinyl acetate,vinyl chloroacetate and vinyl benzoate; vinyl pyridine; vinyl chloride;vinylidene chloride; maleic acid; maleic anhydride; butadiene; ethyleneimine; sulphonated monomers such as vinyl sulphonic acid; styrene andderivatives of styrene such as chloro styrene, hydroxy styrene andalkylated styrenes. A preferred class of comonomers comprisessulphonated monomers such as vinyl sulphonic acid or salts or otherderivatives thereof, such as sodium vinyl sulphonate. Other suitablesalts of vinyl sulphonic acid include the potassium and lithium salts. .The coating preferably comprises 1-60 mole %, more preferably 3-50 mole%, and particularly 5-30 mole % of such sulphonated comonomers in thecoating layer copolymer.

[0041] A preferred coating layer polymer is a poly (acrylic acid covinyl phosphonic acid) copolymer. A further preferred coating layerpolymer is a terpolymer comprising acrylic acid, vinyl phosphonic acidand sodium vinyl sulphonate moieties.

[0042] The molecular weight of the coating layer polymer may vary over awide range but the weight average molecular weight is preferably lessthan 5,000,000, more preferably within the range 2,000 to 200,000,particularly within the range 25,000 to 100,000, and especially withinthe range 35,000 to 80,000. The amount of coating layer polymer presentin the coating layer composition is preferably in the range from 0.1 to50, more preferably 0.1 to 20, and particularly 0.5 to 10 weight %,relative to the total solids of the composition.

[0043] The coating layer composition may additionally comprise a resinwhich is capable of enhancing the adhesion of toner to the coating.Suitable resins include acrylic polymers and copolymers, homopolymersand copolymers of styrene, acrylonitrile, sulphonated polyesters andblends of the afore-mentioned polymers and coploymers. One preferredclass of compatibilizing resins is a copolymer of styrene and acrylicmonomers, for example the resin sold as Neocryl BT70 by Zeneca.

[0044] In one embodiment of the present invention, the coating layercomprises an inorganic filler, suitably particulate materials such assilica, alumina, titanium dioxide and/or metal oxides.

[0045] The inorganic filler particles preferably have a volumedistributed median particle diameter (equivalent spherical diametercorresponding to 50% of the volume of all the particles, read on thecumulative distribution curve relating volume % to the diameter of theparticles—often referred to as the “D(v,0.5)” value) in the range from0.1 to 10 μm.

[0046] Particle sizes of the filler particles may be measured byelectron microscope, Coulter counter, sedimentation analysis and lightscattering, preferably techniques based on laser light diffraction.

[0047] The amount of inorganic filler present in the coating layercomposition is preferably in the range from 0.001 to 30, more preferably<10 weight %, relative to the total solids of the composition.

[0048] The coating layer optionally comprises a, preferably lowmolecular weight, cross-linking agent. The cross-linking agent issuitably an organic material, preferably a monomeric and/or oligomericspecies, and particularly monomeric, prior to formation of the coatinglayer. The molecular weight of the cross-linking agent is preferablyless than 2000, more preferably less than 1500, especially less than1000, and particularly in the range from 250 to 500. Suitablecross-linking agents may comprise alkyd resins, amine derivatives suchas hexamethoxymethyl melamine, and/or condensation products of an amine,e.g. melamine, diazine, urea, cyclic ethylene urea, cyclic propyleneurea, thiourea, cyclic ethylene thiourea, aziridines, alkyl melamines,aryl melamines, benzo guanamines, guanamines, alkyl guanamines and arylguanamines, with an aldehyde, e.g. formaldehyde. A preferredcross-linking agent is the condensation product of melamine withformaldehyde. The condensation product may optionally be alkoxylated. Acatalyst is also preferably employed to facilitate cross-linking actionof the cross-linking agent. Preferred catalysts for cross-linkingmelamine formaldehyde include ammonium chloride, ammonium nitrate,ammonium thiocyanate, ammonium dihydrogen phosphate, diammonium hydrogenphosphate, para toluene sulphonic acid, sulphuric acid, maleic acidstabilised by reaction with a base, ammonium para toluene sulphonate andmorpholinium para toluene sulphonate.

[0049] A further preferred cross-linking agent is dimethylol urea.

[0050] The cross-linking agent preferably exhibits at leasttrifunctionality (i.e. three functional groups) to promoteinter-molecular cross-linking with the functional groups present in thehydroxy alkyl cellulose, and to improve adhesion of the coating layer tothe surface of the underlying layer.

[0051] The amount of cross-linking agent present in the coating layercomposition is preferably in the range from 0.1 to 25, more preferably0.15 to 10, particularly 0.2 to 5, and especially 0.25 to 2 weight %,relative to the total solids of the composition.

[0052] The ratio of coating layer polymer to cross-linking agent presentin the coating layer composition, and consequently in the coating layer,is preferably in the range from 500 to 0.005:1, more preferably 150 to0.01:1, and particularly 50 to 0.1:1 by weight.

[0053] If desired, the coating layer composition may additionallycomprise a surfactant to promote spreading thereof when applied to afilm substrate.

[0054] Furthermore, the coating layer advantageously comprises a drawingagent which preferably comprises an alkylarylphthalate in order tofacilitate processing of film into the desired thickness.

[0055] The surface of the coating layer is hydrophilic, preferablyexhibiting an internal water contact angle, measured as hereindescribed, of less than 70°,more preferably less than 55°, particularlyat most than 50°. The surface of the coating layer preferably exhibitsan oil-in-water contact angle, measured as described hereinafter, ofpreferably greater than 140°, more preferably greater than 145°, andespecially greater than 150°.

[0056] The difference between the water contact angle and theoil-in-water contact angle is preferably greater than 75, morepreferably at least 90 and most preferably greater than 95°.

[0057] The coating layer composition, preferably in the form of anaqueous dispersion, may be applied to the substrate film surface byconventional coating techniques. The applied medium, generally having asolids content in the range from 1 to 30, preferably 2 to 15, andparticularly 5 to 10 weight %, is subsequently dried to remove thedispersant and also to effect cross-linking of the layer. Drying may beeffected by conventional techniques, for example by passing the coatedfilm through a hot air oven. Drying may be effected during normalpost-formation film-treatments, such as heat-setting.

[0058] The thickness of the dried coating layer is preferably greaterthan 0.1, more preferably greater than 0.4 μm, and in particular atleast 1.0 μm.

[0059] The coating layer composition may be applied to an alreadyoriented film substrate. However, application of the coating medium ispreferably effected before or during any stretching operation. Inparticular, it is preferred according to this invention that thehydrophilic coating composition should be applied to the film betweenthe two stages (longitudinal and transverse) of a biaxial stretchingoperation. Such a sequence of stretching and coating is especiallypreferred for the production of linear polyester films, such aspolyethylene terephthalate films, which are preferably firstly stretchedin the longitudinal direction over a series of rotating rollers, coatedwith the coating layer composition and then stretched transversely in astenter oven, preferably followed by heat-setting.

[0060] The reverse surface, remote from the coating layer, of apolymeric film according to the invention may be untreated or may havethereon a functional layer, such as a release layer, a backing layer or,an antistatic layer.

[0061] The polymeric film of the invention may conveniently contain anyof the agents conventionally employed in the manufacture of polymericfilms. Thus, agents such as dyes, pigments, lubricants, anti-oxidants,antistatic agents, surface active agents, slip additives, adhesionimprovers, scratch resistance enhancers, gloss-improvers, prodegradants,fire-retardants, and ultra-violet light stabilizers may be incorporatedin the substrate and/or coating layer, as appropriate.

[0062] The polymeric films may vary in thickness depending on theintended application, but films preferably have a total thickness in therange from 5 to 350, more preferably 25 to 250, and particularly 125 to200 μm.

[0063] In this specification the following test methods have been usedto determine certain properties of the filler particles and polymericfilm:

[0064] Contact Angle

[0065] Water contact angles were obtained by photographing the profilesof 5 μl droplets of distilled water test liquids on the sample surface.The angles were measured by projecting the photographic negative anddrawing the tangent to the droplet profile at the point of three phasecontact. The contact angles quoted with standard deviations are the meanof the angles measured for 9 drops of each liquid. The standarddeviation was 4.

[0066] Oil-in-water Contact Angle

[0067] The contact angle of a mineral oil (Castrol Solvent Neutral 150)on the test surface submerged in a water environment, was measured byinverting the sample test surface in an glass cell (60×50×55 mm)containing distilled water, and introducing a droplet of the oil (10-40,μl) onto the underside of submerged surface by means of a syringe andcurved needle. Images of the drop profile were captured using theFTA-200 Dynamic Contact Angle System and the contact angle automaticallycalculated using the instrument software. The contact angles quoted arethe mean of 9 drops. The standard deviation was 4.

[0068] Filler Particle Analysis Volume distributed median particlediameter, and particle size distribution ratios D₂₅/D₇₅ and D₁₀/D₉₀ weremeasured using a Coulter LS 130 (Coulter Electronics Ltd. Luton, UK)particle sizer.

[0069] BET specific surface area was measured by multi-point nitrogenadsorption using a Micromeritics ASAP 2400 (Micromeritics Limited,Dunstable, UK). Relative pressures between 0.05 and 0.21 were used, andthe outgassing conditions were 1 hour at 140° C. with nitrogen purge (1to 2 litres/hour).

[0070] Skeletal density was measured by helium pycnometry using aMicromeritics Accupyc 1330 (Micromeritics Limited, Dunstable, UK).

[0071] Adhesion to toner

[0072] The adhesion of toner to the hydrophilic coating was estimated bymeasuring the amount of toner removed from the coating by the followingmethod:

[0073] A test image (of toner) was printed onto a sample of the coatedfilm using a Xante laser printer. The test image consisted of eightblack rectangular blocks. The optical density of each of the eightblocks was measured using a color transmission-reflection densitometer(model Mackbeth TR927, supplied by Optronic Colour Communications). Thehighest and lowest measured values were disregarded and the averagevalue of the other six values was recorded as V₁. Toner from one blockwas then removed by applying a piece of adhesive tape (Tesa 4104) andremoving the tape in a standard way. Eight further densitometermeasurements were then taken of the area from which the toner had beenremoved and the average value (disregarding the highest and lowest) ,V₂,was compared with the previous average to calculate the % of tonerremoved by the following equation:

toner removed=(V ₁ /V ₂)×100%.

[0074] The invention is illustrated by reference to the followingexamples.

[0075] Experimental Method and Materials

[0076] The materials used to make up the coating formulations describedhereinafter are detailed in Table 1. The composition of each formulationis given in Table 2.

EXAMPLES 1 and 2

[0077] A clear polyethylene terephthalate polymer was co-extruded with apolyester copolymer made from 18% isophthalate+82%terephthalate+ethylene glycol, cast onto a cooled rotating drum andstretched in the direction of extrusion to approximately 3 times itsoriginal dimensions. The copolymer side of the cooled stretched film wasthen coated by means of offset gravure coating using a rubber applicatorroller, with an aqueous coating composition containing the followingingredients in specified quantities. TABLE 1 VPA Vinyl phosphonic acidsupplied by Albright & Wilson. AAVPA 877 poly (acrylic acid co vinylphosphonic acid) 70:30 mole % supplied by Albright & Wilson AAVPA 1014poly (acrylic acid co vinyl phosphonic acid) 30:70 mole %, Mw approx60,000, supplied by Albright & Wilson UK) used as 10% w/w aqueoussolution AAVPA 10 15 poly (vinyl phosphonic acid co vinyl sulphonicacid) 10:90 mole % suppliedby Albright & Wilson, UK; used as 10% w/waqueous solution AAVPA 1021 poly (acrylic acid co vinyl phosphonic acidco vinyl sulphonic acid) 40:50:10 mole %, Mw approx 60,000, supplied byAlbright & Wilson, UK; used as 25% w/w aqueous solution aptsa ammoniumparatoluenesulphonic acid (x-link catalyst) used as 10% w/w aqueoussolution Cymel 350 Methylated melamine formaldehyde cross-linking agent,obtained from Dyno-Cyanamid K.S., used as 10% w/w aqueous solution DMUDimethylol urea obtained from Aldrich, used as 10% w/w aqeuous solutionNeocryl BT-70 acrylic-styrene copolymer emulsion, supplied by Zeneca;used as 19% w/w aqueous solution Seahostar a mono ethylene glycol/silicamixture, obtained from KE-70 Nippon Shokubai Co. Ltd; used as 20% w/waqueous solution Synperonic an alkyl nonylphenol ethoxylated surfactantsupplied by NP 10 Imperial Chemical Industries; used as 10% w/w aqueoussolution Santisizer 261 Iso-Octyl benzyl phathlate supplied by Monsanto;mico-emulsion used as a 2% w/w aqueous solution Tospearl 120 2 μmparticles of a cross-linked polysiloxane filler, obtained from ToshibaSilicone Co Ltd; used as 2% w/w dispersion in ethylene glycol Tospearl344 4.5μ particles of a polysiloxane filler, supplied by ToshibaSilicone Co Ltd. Glycerol Milease T Polyester resin supplied by ICIAmericas

[0078] The polyester film was coated on one side only. The coated filmwas passed into a stenter oven, where the film was dried and stretchedin the sideways direction to approximately 3 times its originaldimensions. The biaxially stretched coated film was heat set at atemperature of about 200° C. by conventional means. Final coatingthickness was 0.03-0.05 μm with a coat weight of approximately 0.3 to0.5 mgdm⁻². The surface characteristics of the coated films and adhesionof toner were tested as described above and the results are given inTable 3.

Example 3

[0079] A coating formulation as shown in Table 2 was prepared and thefilm coated by the same method as described in Examples 1 and 2. Thecoating was applied at three different coat weights to give driedcoating thicknesses of 2.8, 1.2 and 0.7 μm. The water contact angleswere measured and are given in Table 3. The results show that a thickerfilm has a lower water contact angle and is therefore more hydrophilic.

[0080] The Santisizer 261 is a drawing agent and is made into, and usedas, a 2% micro-emulsion. To prepare the micro-emulsion used in Examples7 and 8:

[0081] Add 200 ml of neat Santisizer 261 and 800 ml neat Synperonic NP10to a clean plastic bottle. Screw on the lid and shake well to produce aclear, one phase, liquid. Pour slowly into a vessel containing 9 litresof deionized water with stirring. Allow to stir for 10 minutes, thenstand to allow foam to disperse.

Examples 4-9

[0082] Coating formulations as shown in Table 2 were prepared and thefilm coated by the same method as described in Examples 1 and 2.

[0083] The results for the water contact angles given in Table 3 showthat addition of a drawing agent such as glycerol, in particularSantisizer 261 substantially decreases the water contact angle. TABLE 2Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7Example 8 Example 9 % w/w % w/w % w/w % w/w % w/w % w/w % w/w % w/w %w/w VPA 1.2 AAVPA 877 2 AAVPA 1014 8 2 AAVPA 1021 8 2 2 2 AAVPA 1015 2aptsa 0.03 0.03 0.2 0.2 0.2 0.2 0.2 0.2 Cymel 350 0.43 0.43 8.3 DMU 2 22 2 2 2 Neocryl BT-70 1 1 Seahostar KE-70 0.02 0.02 Synperonic NP10 0.20.2 0.2 0.2 0.2 0.2 0.2 Santisizer 261 0.1 1 micro-emulsion Tospearl 1200.01 0.01 Tospeal 344 24 Glycerol 4 4 4 4 Milease T 4.7 demineralisedwater 90.5 90.5 91.6 91.6 91.6 91.6 95.77 94.8 61.6

[0084] TABLE 3 Water Oil in (Oil in Water Contact Toner Contact WaterContact Angle) - (Water Removed Angle° Angle° Contact Angle) % Example 150 145  95 24 Example 2 39 150 111 44 Example 3 23 142 125 (2.8μ) (1.2μ)28 (0.7μ) 54 Example 4 44 Example 5 14 Example 6 26 Example 7 12 Example8 <1 Example 9 7.4

1. A polymeric film which is substantially gelatin free and comprises apolymeric film substrate having on at least one surface thereof, acoating layer comprising a polymer comprising at least one or morerepeating units comprising at least one or more pendant (—POXY) groups,wherein X and Y, which may be the same or different, are OH or OMwherein M is a cation.
 2. A film according to claim 1 wherein saidcoating layer polymer comprises repeat units containing pendantphosphonic acid groups and/or salts or other derivatives thereof.
 3. Afilm according to claim 1 or claim 2 wherein said coating layer polymercomprises a copolymer comprising one or more acrylic comonomers.
 4. Afilm according to any one of the preceding claims wherein said coatinglayer polymer further comprises repeating units containing pendantcarboxyl groups or groups which are capable of forming carboxyl groupson hydrolysis.
 5. A film according to claim 4 wherein said coating layerpolymer comprises a copolymer of acrylic acid and vinyl phosphonic acid.6. A film according to any of the preceding claims, wherein said coatinglayer polymer further comprises a comonomer which is a sulphonatedmonomer or a salt or other derivative thereof.
 7. A film according toany of the preceding claims wherein said coating layer polymer comprisesa copolymer of acrylic acid, vinyl phosphonic acid and sodium vinylsulphonate.
 8. A film according to any one of the preceding claimswherein said coating layer further comprises a drawing agent.
 9. A filmaccording to claim 8 wherein the drawing agent comprises analkylarylphthalate.
 10. A film according to any one of the precedingclaims wherein said coating layer polymer further comprises a filler.11. A film according to any one of the preceding claims wherein saidcoating layer polymer further comprises a cross-linking agent.
 12. Amethod of producing a polymeric film which comprises forming a polymericfilm substrate, applying a coating composition to at least one surfaceof the substrate, the coating composition comprising a polymercomprising at least one or more repeating units comprising at least oneor more pendant (—POXY) groups, wherein X and Y, which may be the sameor different, are OH or OM wherein M is a cation.
 13. A method accordingto claim 12 wherein the coating composition is applied to the substratebefore or during any stretching operation employed to effect molecularorientation of the film substrate.
 14. A printing plate comprising apolymeric film substrate having on at least one surface thereof, acoating layer comprising a polymer comprising at least one or morerepeating units comprising at least one or more pendant (—POXY) groups,wherein X and Y, which may be the same or different, are OH or OMwherein M is a cation.